The present invention relates to a process for producing rigid polyurethane foams having a thermal conductivity of less than about 0.130 Btu-in./hr.ft.sup.2.F and to the foams produced by this process.
Rigid polyurethane foams and processes for their production are well known in the art. Such foams are typically produced by reacting a polyisocyanate with an isocyanate-reactive material such as a polyol in the presence of a chlorofluorocarbon blowing agent. It is also known, however, that these chlorofluorocarbon blowing agents pose environmental problems.
Alternatives to the known chlorofluorocarbon blowing agents are currently the subject of much research. Hydrogen chlorofluorocarbons (HCFC) are among the most promising alternatives. However, some HCFCs must be used in larger amounts than the traditional chlorofluorocarbons and often result in foams having thinner cell walls. The HCFCs also tend to migrate from foam cell cavities thereby reducing the insulation value of the foam. Further, HCFCs produce a more thermally conductive foam insulation which reduces the energy efficiency of appliances, e.g., which are insulated with such foams. Consequently, substitution of HCFCs for the traditional chlorofluorocarbons may resolve the environmental problems created by chlorofluorocarbons but it creates an energy efficiency problem. It would therefore be advantageous to develop a process for producing energy efficient, rigid polyurethane foams in which none of the traditional chlorofluorocarbon blowing agents was employed.
One approach to resolving the migration problem of HCFCs has been to use mixtures of the traditional chlorofluorocarbons and the HCFCS. This approach is disclosed in U.S. Pat. Nos. 4,927,863 and 4,945,119. These blowing agent mixtures do, however, include some of the undesirable traditional chlorofluorocarbons.
U.S. Pat. No. 4,943,597 discloses a process for producing rigid polyurethane foams in which water is used as the blowing agent. Other optional blowing agents disclosed in this patent include the known low boiling halogenated halocarbons and "azo" blowing agents.
U.S. Pat. No. 4,960,804 discloses a process for producing rigid foams in which a mixture of an HCFC and an alkyl alkanoate is used as the blowing agent.